Novak Djokovic had to come from behind to beat Kei Nishikori and keep his bid for a calendar Grand Slam alive as he advanced to the US Open fourth round. The world number one overcame a loose first set to win 6-7 (4-7) 6-3 6-3 6-2 and beat the Japanese for the 17th time in a row. The Serb, 34, is aiming for a men's record 21st Grand Slam title. He will face American Jenson Brooksby next, after the wildcard beat Russian 21st